Braking distribution system for a multi-axle vehicle making allowance for background braking

ABSTRACT

An electronically controlled braking system wherein improved braking distribution in a multi-axle vehicle is achieved by making allowance for sources of background braking by assessing through measurement the total background braking force and assigning this in a predetermined proportion between the vehicle axles.

BACKGROUND OF THE INVENTION

The present invention relates to a means for obtaining improved brakingdistribution in vehicle braking systems, whether electronicallycontrolled or mechanical.

The curve governing the relationship between axle braking for a two-axlevehicle is well known but does not take into account other sources ofbraking which would cause vehicles to come virtually to rest withoutfriction brakes being employed. Such other sources of braking areprimarily frictional losses in rotating parts and, more importantly,engine braking. Many heavy vehicles have means of increasing enginebraking or have some form of retarder, in the form of an additional typeof endurance brake which acts on the driving wheels. In such vehicles inparticular, the use of combined braking seriously unbalances the brakingdistribution away from the ideal in which utilization of adhesionbetween axles is equal. In conventional vehicles, efforts are made toobtain good distribution of foundation (friction) braking but theadditional braking sources generally represent an option or anafterthought and are in no way integrated into an overall braking systemfor the vehicle.

In a Brake-by-Wire or Electronic Braking System, electronic controls areprovided in the combined braking scheme which, of course, useselectrical signalling, in order to make braking distribution near to theideal so as to improve safety in braking. Such a system has therefore toinclude some level of integration of the sources of braking. This may bethe full integration of a blended braking system or the lesserintegration of an interlinked system in which the foundation brakecontrols make suitable allowance for other sources of vehiclebraking--referred to hereinafter as background braking--in order toachieve a more ideal braking distribution in actuality. It is thislatter type of braking system which is the subject of the presentinvention.

It is recognised as impossible to measure all the sources of backgroundbraking because the sources are distributed and often quite small and,as such, sensors are not available to measure the effects of what isoften losses and subject to relatively significant disturbances duringvehicle movement. However, it is possible to measure some of the sourcesof background braking directly, such as driveline torque. It is alsopossible to measure the combined effect of all the sources of backgroundbraking in the deceleration which is produced on the vehicle and from astudy of typical vehicles to allocate this effect between axles.

SUMMARY OF THE INVENTION

Thus, in accordance with the present invention in its broadest aspect,there is provided a method and apparatus for achieving improvements tobraking distribution in a multi-axle vehicle having a controlled brakingsystem, wherein allowance is made for selected sources of backgroundbraking by assessing through measurement the corresponding backgroundbraking force and assigning this in a predetermined proportion betweenthe vehicle axles.

In one embodiment, for example, the selected source can be the drivelinetorque which can be measured directly and the corresponding backgroundbraking force assessed.

However, it is presently preferred to assess the total backgroundbraking force by its resultant effect on the vehicle performance, namelyits decelerating effect. This assessment can therefore be made bymeasurement of the deceleration of the vehicle at a time when thevehicle is not being driven forward and the foundation brakes are notbeing applied, that is, by measurement of the total vehicle rollingdeceleration.

The deceleration effect is measured using a vehicle borne decelerometer,which is zeroed when level in order to obtain a figure which iscompensated for gradient changes.

Of the background braking contribution which is thus allocated betweenaxles, the majority is assigned to the drive axle (usually the rear axlein the case of trucks and heavy vehicles), and the equivalent brakingpressure is calculated from the brake factors and the total vehiclemass. These equivalent braking pressures are registered as thedistributed background braking levels and are allowed for during thefoundation braking application. This automatically accounts for abackground offset when the vehicle achieved deceleration and thedriver's demand are compared, so that in an adaptive control system,such as that described in our European Patent EP 0205277, the backgroundbraking is prevented from wrongly adapting the brake pressure constant.

The background braking level is preferably measured from a filtereddeceleration signal which, because of the time constant of the filter,typically of the order of 2 seconds, is slightly delayed and not subjectto serious disturbances resulting primarily from suspension movements.This signal is generated continuously and is preferably sampled justafter the brake pedal is pressed so that it represents the backgroundretardation just before foundation braking commences. The signal isnormalised to a preset maximum speed and stored for use throughout thestop. As the speed falls, the deceleration caused by background sourcesalso falls and a typical fall off curve is programmed into the brakesystem controller. Thus at each pressure setting cycle, a decayingbackground deceleration can be calculated and used as follows:

a. The calculated deceleration is split into 2 axle components after itis converted into an equivalent braking force by multiplying it by thetotal vehicle weight. The front axle component is formed by taking apreset small percentage of the total braking force and the remainder isassigned to the rear axle.

b. Each axle force is related to an equivalent braking pressure from thefollowing expression ##EQU1## where FBA=Axle Braking Force

KBA=Axle Brake Const Force/Bar

c. The background retardation equivalent axle pressures are assumed tobe acting before the application of foundation braking and so, whenbraking pressures are calculated, they are reduced by components PB_(f)and PB_(r) before being applied to the pressure control loops.

Thus, in a preferred arrangement in accordance with this invention, thebackground braking level is arranged to be reduced in a pre-programmedmanner as the vehicle speed falls. Furthermore, the measurement ofbackground vehicle deceleration is preferably made just after the firstpedal movement whenever the foundation brakes are applied, ant storedfor use throughout the stop.

Advantageously, the measured figure is normalised to a predeterminedpreset maximum speed for the vehicle and this stored figure on each stemis reduced as the vehicle speed falls on the basis of a speed fall-offratio table.

In one preferred arrangement, falling background deceleration isconverted into braking force, allocated between axles in apre-programmed ratio and converted into background braking pressureswhich are subtracted from each corresponding axle braking pressurecalculated to meet the driver's braking demand.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described further hereinafter, by way of example only,with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic illustration of a typical electronicallycontrolled braking system (EBS) to which the present invention can beapplied;

FIG. 2 shows a filtered deceleration signal showing part of a step fromwhich the background deceleration can be measured;

FIG. 3 is a record of typical deceleration variation with speed for astop on level road;

FIG. 4 is a flow chart which shows the derivation and storage of anormalised background deceleration figure;

FIG. 5 is the flowchart which shows the equivalent background pressurecalculation and how this links with the main pressure calculationroutine which is continuously performed at defined intervals;

FIG. 6 is a block diagram illustrating one embodiment with the presentinvention; and

FIG. 7 is a block diagram illustrating a second embodiment of a vehiclebraking system in accordance with the present invention; and

FIG. 8 is a diagrammatic illustration of one way in which the presentinvention can be applied to a mechanical braking system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The background to the development of the present invention is firstconsidered.

The fundamental basis of every past and new vehicle calculationregarding braking or traction has been and currently still is the "idealtangential force diagram", based on the publication by Tonnies, C., ATZ1955, Issue 8 in 1955, v. Glassner published on the subject in 1973.This diagram describes what it says, that is an ideal condition ofdynamic load transfer between the axles, caused by ideal and purebraking forces by the wheel brakes, or traction forces acting on thevehicle.

Current practice is that a small family of extreme dynamic borderlineconditions are selected, based on their minimum and maximum loaddistribution, together with their assumed heights of centre of gravity,in order to represent all dynamic vehicle situations in straight linebraking and traction. With this information, the relevant idealtangential force diagram is calculated and drawn.

Into this family of ideal dynamic tangential force curves a fixed,linear braking distribution is calculated, in most cases with reducingor limiting value functions at different cut in points.

The gap between the fixed lines of braking distribution and the relevantideal curves indicates the loss in tire to road utilisation, i.e. thefront or rear wheels lock earlier than the coefficient of adhesion theyare rolling on. Utilisation influences stopping distance; stoppingdistance increases linearly with reduced utilisation.

In order to utilise the tire to road friction coefficient fully and toachieve therefore the shortest possible stopping distance, when needed,the aim must be to obtain, on all wheels simultaneously, a tire to roadutilisation as close to 100% as possible. The implications of this are:

1. A non linear, "fixed" braking distribution has to be used.

2. The "fixed", non linear braking distribution must be adaptable tochanging vehicle and road conditions.

It has been noted during vehicle testing, that certain test results ofmaximum achievable vehicle deceleration did not comply with data sheetpredictions based on the conventional ideal tangential force curve. Thebiggest discrepancies are during tests with Anti Lock Braking systems onindividual axles. Utilisation values in excess of 125% can be calculatedduring such tests made according to current regulations.

However, any utilisation value in excess of 100% clearly indicatesinherent flaws of the theory and strategy in use.

The ideal tangential braking force curve does not take intoconsideration other factors, which cause vehicle retardations. Inaddition to the working brakes, there are: engine braking; retardersystems; exhaust braking; downwards gear changing; frictional bearinglosses; residual torque due to dragging brakes; losses due to tire work;different acting ratios between front and rear axle on four wheel drivevehicles; different axle loading due to aerodynamic forces; anddifferent threshold pressures between front and rear brakes, resultingin premature braking of one axle, or even one brake during brakeapplication, etc.

A number of attempts have been made over the years (v. Glassnet, E. C.Dissertation, UNI Stuttgart, 1973 and Horz, E./Illg, V., The tangentialbraking force diagram and its use in the reconstruction of trafficaccidents, DAT-Seminar, 1984, Frankfurt) to solve this task, butpractically all of them have tackled the problem by parallel shift ofthe coordinate system for the fixed braking distribution in relation tothe ideal tangential braking force curve. It has been assumed in theseattempts, that the changing conditions of a vehicle have to be describedin a number of different coordinate systems with different points oforigin. It has been furthermore assumed, that other retardation forces,e.g. forces caused by engine braking, are linear constants in relationto the ideal tangential braking force curve, and have no influence onthe ideal curve itself.

All previous proposals simply change the position of the linear, fixedbraking distributions within the coordinate system and compare it withthe ideal tangential braking force curve for effectiveness.

Therefore a new strategy has been sought and developed by the presentApplicants based on straight line braking, which reflects all possiblevehicle conditions. The new strategy builds on and takes into accountthe existing tangential braking force diagram and uses the samecoordinate system.

As a baseline it is important to review the relevant equations for theknown "ideal tangential braking force" diagram.

In equation 1, front axle braking force over vehicle weight is given by##EQU2## where k equals z in numerical value. Equation 3. is valid forthe rear axle braking forces over vehicle weight, ##EQU3## where kequals z in numerical value.

The formulae 1., 2., 3. and 4 are describing an ideal tangential brakingforce curve, which obviously is a parabola. This parabola represents theideal case of maximum and identical tire to road utilisation on all ofits points, with the achievable vehicle deceleration being identical tothe available tire to road coefficient of adhesion.

What has been missing up to now in current practice is the reflection inthe formulae of the practical disturbances, such as engine braking,retarder forces, frictional losses etc. All of those factors causevehicle retardations, and their recognition in the theoretical basis forfuture, optimised braking systems is a primary aim of the presentinvention.

Any braking or traction system is only as good as its theoretical basiswill permit. A largely optimised braking system is only possible andfeasible, if the braking STRATEGY in use is CORRECT.

It is essential to understand that vehicle decelerations may be createdby all sorts of individual forces, some of which are independent of thebrakes and possibly act in parallel or in addition.

One may consider, by way of example, the following scenario:

A rear axle driven vehicle is cruising at speed on a country road.

Both axles, front and rear, have different, qualifiable frictionallosses in their axle bearings.

The tires on the front axle are inflated to a different pressure thanthe rear tires and generate therefore a different rolling resistance dueto different tire work per axle.

The driver takes his foot off the accelerator pedal and applies, via ahand valve, a hydraulic or electromagnetic retarder.

Whilst he has taken his foot off the accelerator pedal, the vehiclespeed may reduce due to retardation via gear box and engine braking.

The retarder system installed acts via drive shaft and differential ontothe rear axle and causes vehicle deceleration.

The vehicle may have no retarder system installed, and the driverdecides to reduce the vehicle speed by changing down in gear.

Immediately after the gear change the brake pedal may be applied by thedriver, in order to bring the vehicle to rest, or

The driver has to apply his brakes in an emergency because of sometraffic condition.

Certainly this is a complex situation but it is nevertheless a day today traffic condition probably known to practically all drivers. Theimportant point is, that all of the listed conditions have one seeminglyunrecognised fact in common:

All of the conditions cause "other" inner dynamic forces, which reducethe vehicle speed. Reduction in vehicle speed is caused by vehicledeceleration. Vehicle deceleration implies dynamic weight transfer.

Dynamic weight transfer on road vehicles is therefore not just caused byconventional braking forces, but is caused as well by a number of otherbackground forces. "True optimal" tangential braking force distributionis a novel term of definition, which provides a better distinction whencompared to the misleading expression "ideal tangential braking forcedistribution". The "true optimal distribution" is the dynamic tangentialbraking force distribution, which reflects all individual decelerationsacting on the vehicle.

The true optimal tangential braking force distribution distinguishes indeceleration terms between:

ΔZ→vehicle deceleration via front and rear axle, caused by all wheelbrakes applied in parallel,

z_(i).1 →vehicle deceleration via the front axle, caused by inner forcesother than the purposely applied brakes,

z_(i).2 →vehicle deceleration via the rear axle, caused by inner forcesother than the purposely applied brakes,

All inner retardation forces on the vehicle other than the purposelyapplied brakes, for example from retarders, rolling resistance, gearchange, engine or exhaust brake, dragging brakes etc., can be assignedto either the front or rear axle. These front or rear axle forces willcause vehicle decelerations, which are the ones referred to in thestated vehicle decelerations z_(i).1 via the front axle and z_(i).2 viathe rear axle.

These research results can be described in generic form by the followingnovel dynamic equations 5., 6., 7. and 8. :

Equation 5. describes the true optimal FRONT AXLE braking force overvehicle weight, ##EQU4## Equation 7. is valid for the true optimal rearaxle braking force over vehicle weight, ##EQU5##

The tire to road surface coefficient of adhesion k, on which the totalvehicle is moving with both front and rear axle, is now numericallyidentical with the sum of braking force deceleration plus inner forcedeceleration of the vehicle.

For a two axle road vehicle, being braked in parallel on front axle andrear axle according to the true optimal tangential braking forcedistribution, the total coefficient of road to tire adhesion istherefore defined by:

k→total vehicle coefficient of road to tire adhesion

    k=(z.sub.i.1 +z.sub.i.2 +Δz)                         9.

and

    k=z                                                        10.

This definition makes sense. If, for example, a gear shift decelerates arear axle driven vehicle with a defined deceleration, which is part ofz_(i).2, together with an additional front axle rolling resistance ofz_(i).1 on a given coefficient of adhesion k, the remaining vehicledeceleration due to and required from the brakes is z, in order to reachthe locking point at k. Any larger deceleration demand from the brakeswould overbrake and lock the rear axle in this case. The total vehicledeceleration can not be larger than the total available coefficient oftire to road adhesion of the vehicle.

The vehicle deceleration caused by each individual axle during thebraking process of the vehicle may be different between front and rearaxle. The individual axle may contribute therefore differently to thetotal vehicle deceleration.

The individual coefficient of adhesion, utilised by each axle in thetrue optimal tangential braking force diagram, can therefore be definedas k₁ →utilised coefficient of adhesion by the front axle

    k.sub.1 =(z.sub.i.1 +z)                                    11.

and

k₂ →utilised coefficient of adhesion by the rear axle

    k.sub.2 =(z.sub.i.2 +z)                                    12.

Nevertheless, this condition is not to be mixed up with the "split k"condition, during which the one side of both front and rear axle of thevehicle is being braked on a different coefficient of adhesion than theother side. In the "split k" case the locking points of the wheels pervehicle side are different, but the functional line of the true optimaltangential braking force distribution remains unchanged. The brakingpart of the true optimal tangential force curve in the 1. quadrant ofthe coordinate system with braking force front axle over vehicle weightB_(F) /P on the abscissa and braking force rear axle over vehicle weightB_(R) /P on the ordinate is now clear.

The true optimal tangential force diagram describes on all the points ofits family of curves the condition, where the coefficient of adhesionbetween tires and road surface equals the maximum transmittable vehicleretardation or acceleration.

The vehicle in the quadrant for braking is moving and dynamic. All ofthe described "inner forces" including the application of the brakes,all the rolling resistance etc. tend to decelerate the vehicle andtherefore reduce the vehicle speed. That is the reason why a number ofdynamic forces can be identified, which are responsible for the vehicledeceleration in line with the novel formulae.

In the case of vehicle traction (as compared to deceleration), thevehicle may be already moving or it may be stationary, when the driverintends to accelerate the wheels, in order to move the vehicle. None ofthose forces described in the previous braking part can accelerate thevehicle in addition to the tangential traction forces on the rollingradius of the tires, created at the contact patch between tires and roadsurface by engine, gear box and transmission. Therefore none of theseforces have to be considered in the true optimal tangential forcediagram in its traction part.

The proposed formulae 5, 6, 7 and 8., which describe the true optimaltangential braking force distribution curve, consider all vehicledecelerations caused by inner forces, which are attributable to thefront or rear axle.

Turning now to FIG. 1, there are illustrated, inner alia, the maincomponents of a conventional electronic braking system (EBS) which isnot described in detail herein. Driver's braking demand signals aregenerated electrically by a treadle-operated transducer arrangement 10and supplied to an electronic controller 12 where front and rear brakingpressures are established and fed to front and rear brake actuators 14,16 for the "foundation brakes" via respective relay valves 18, 20. Theresulting foundation braking pressures depend upon operating parametersof the vehicle determined, inter alia, by front and rear load sensors22, 24, a vehicle decelerometer 26 and a speed sensor 28.

For the purposes of the present technique, the "background brakingforces" are assembled from the vehicle deceleration, as measured in theparticular embodiment by the decelerometer 26, when the vehicle is notbeing driven forward and not yet being braked by the foundation brakes14, 16. The preferred point in time at which to make this backgrounddeceleration measurement is just at the point where foundation brakingis signalled but before the brake pressures have caused brake forces tobe developed. Deceleration measurements are made continuously and arefiltered in order to remove noise and this process generates a delaywhich depends on the time constant of the filter used. With a 1-2 secondtime constant, for example, the decoloration reading is slightly delayedso that if a sample is taken of the filtered signal as the brakingdemand signal starts to build up the figure obtained will be thebackground deceleration which existed just before the pedal was pressed.This, as shown in FIG. 2, is the sample which is stored along with avehicle speed reading and is used to form the background braking effortwhich will be used throughout the stop. This background effect does not,however, remain as a constant force but falls with vehicle speed. At agiven speed it is also not constant from stop to stop as it dependsprimarily on which gear is engaged and this is why a starting samplemust be taken.

FIG. 3 shows a typical deceleration fall-off in relation to vehiclespeed, obtained in a series of vehicle trials. The graph of fall-offratio is programmed into the braking controller and is used to predictthe deceleration at any speed which results from background braking. Touse the ratio, the measured deceleration figure, which is sampled andstored along with the current vehicle speed is normalised to a maximumspeed value. This is achieved by looking up the speed factor in a table,which is a digital replica of FIG. 3, and dividing the storeddeceleration figure by this factor to give the equivalent decelerationat 150 kmph in the example shown. As this calculation is made and theresult is stored as a normalised deceleration, a flag is set to showthis and subsequent reference to this figure is made along with afalling ratio factor as the vehicle speed reduces. The process ofsampling and normalising is illustrated in FIG. 4 which is executed onceonly at the start of each stop. The flag which is set in the routine isreset at the end of each stop when the brake pedal is restored.

The calculation of equivalent background pressures and then the axlebraking pressures are shown in the flow chart of FIG. 5 which isexecuted many times per second in order to set the braking pressures inresponse to what may be changing driver demands. The equivalentbackground braking pressures are calculated by looking up a fall-offfactor based upon the current vehicle speed from the stored table andmultiplying this by the stored normalised background decelerationcomponent which can reasonably be assigned to background sources. Thiscomponent which reduces as the speed falls is divided into front andrear axle components after it is converted into an equivalent brakingforce FBK by multiplying by the total vehicle weight. A fixed constantsmall fraction of this force FBK is formed and subtracted to representthe front axle losses and the remainder is assigned to the rear axleusing preset brake constants for each axle. The forces are converted toequivalent background braking pressures by dividing them by brakeconstants for each axle. These pressure results are stored and laterwill be used to reduce the calculated foundation braking pressures. Thisprocess is skipped if the "Normalised Background Deceleration Stored"flag is not set so that the routine is only executed during braking. Toset braking pressures, the pedal demand is read and zero-corrected andthe route is bypassed if there is no braking demand with axle pressuresbeing held at zero. When braking is demanded, the product of demand andaxle load (dynamic) is formed and further multiplied by the pressureconstant which scales the result into pressure terms. Thus weightsensitive braking pressures are established for each axle, theappropriate background pressures developed in the previous routing aresubtracted and the resulting somewhat reduced pressures are output tothe axle pressure control loops.

Referring now to FIG. 6, there is shown by way of illustration a firstembodiment of a braking system utilising the technique of the presentinvention. This embodiment operates in accordance with the flow chart ofFIG. 5 and assesses the total background braking force by measuringvehicle deceleration when the vehicle is not being driven forward andnot yet being braked by the foundation brakes.

This system sets and controls braking pressures in response to driverdemands by the use of two pressure control hoops 30 and 32, for thefront and rear brakes respectively. Each pressure control loop 30, 32takes an electrical input signal D from a brake pedal transducer 34which is used to provide a pressure error signal E by comparison withthe output signal P1 of a pressure transducer 36, this pressure error Eforming the input to a computer-based pressure controller 38 whichgenerates an output signal causing the pressure developed by anelectro-pneumatic or electro-hydraulic converter 40 to change in adirection such as to reduce the amplitude of the pressure error E. Theconverter 40 is supplied by a pneumatic or hydraulic pressure supply 42,as appropriate. The pressure controller 38 employs a pair of solenoidvalves 44a, 44b to raise or lower a control chamber pressure byselective energisation of these valves. The converter 40 operatespneumatically in this instance and employs a relay valve 46 whichresponds to this control chamber pressure and which re-balances into theclosed condition when the brake pressures at brake actuators 48a, 48bfor left and right-hand brakes 50a, 50b become equal to said controlpressure. The aforegoing arrangement of the pressure control loops 30,32 is well known and needs no further explanation here.

The input demand signal D is also supplied to a pulse generator 52 whoseoutput is passed to one input of a pair of AND gate 54, 56. The otherinput of the AND gate 54 is connected to the output of a decelerometer57 which is responsive to the overall deceration of the vehicle. Theoutput of the AND gate 54 is connected to a store 58, which alsoreceives the output of the deceierometer 56 direct. This arrangementenables the deceleration, prevailing immediately before a braking demandis initiated, to be measured and stored. The other input of the AND gate56 is connected to a vehicle speed sensor 60 so that the prevailingvehicle speed is passed, via the AND gate 56, to a normalisation block62, which also receives the speed signal direct and the storeddeceleration from the store 58, to enable the normalisation step to beeffected wherein the measured deceleration figure is normalised to amaximum speed value. This is achieved using a digital look-up table 64which stores a fall-off curve 65 which is normalised for the maximumvehicle speed. The resulting equivalent deceleration is converted in abackground force converter 66 into a signal level B representative ofthe total background braking effort. In one embodiment, the decelerationassessment is made from the average of readings taken over a preset timeperiod historical to the point at which the brakes are applied.

From this signal B, allowance is made for the distribution between frontand rear axles in proportion to a preset constant fraction held in theproportioning device 68 (F/R). The proportioning device 68 produces afirst value of background force to be assigned to the front axle and asecond value of background force to be assigned to the rear axle, eachvalue being a predetermined fraction of the whole background brakingforce B. The first value of front background braking force is fed to thefront brake factor converter 70 to produce an equivalent front brakepressure to be subtracted from the front brake control demand insubtractor 72a. The second value of front background braking force isfed to the rear brake factor converter 76 to produce an equivalent rearbrake pressure to be subtracted from the rear brake control demand insubtractor 78.

The system further includes a gear selector element 80 responsive toengine speed 82 and road speed 84 for selecting an appropriate gear toprovide a signal GS representative of the selected gear. The gearselector 80 could, alternatively, be operated manually. The signal GS ispassed to a multiplying element 86 in the backaround force converter 66which provides a multiplying tactor for shifting the curve provided bythe look-up table in dependence upon the selected gear. In analternative arrangement, which is more expensive and so less preferred,the signal GS is passed directly to the look-up table block 64 so as toselect a different one of a plurality of stored curves, dependent uponthe selected gear.

The operation of the aforegoing system follows the flow chart of FIG. 5.

Whereas the latter system measures the total background decelerationeffect by all sources, an alternative strategy is to separately measureone, several or all individual sources of such background deceleration.One principal factor which can be measured directly is the drive linetorque, measured for example by a torque transducer positioned aroundthe vehicle prop shaft. Such a system is illustrated in principle inFIG. 7.

The system of FIG. 7 again uses the two pressure control loops 30,32 forthe front and rear brakes fed with demand signals from the brake pedaltransducer 34. Equivalent parts in FIGS. 6 and 7 are given the samereference numerals.

In this case, the output of the pulse generator 52, which indicates thepresence of a braking demand D, is passed to one input of an AND gate90, whose other input comes from the aforementioned torque sensor 92mounted around the prop shaft. The output of the AND gate 90, and theoutput of the torque sensor itself, are passed to a unit 94 whichprovides a signal T representative of the prevailing driveline torquelevel. This is converted in the element 76 into a rear brake factor RBFfor subtraction from the demand signal D before it is applied to thepressure control loop 32 for the rear brakes. In this case, no reductionof the measured driveline torque is made to allow for braking effects(very small) specifically associated with the front axle.

The latter system has the advantage that, since the signal used togenerate the rear brake factor RBF is generated continuously during abraking operation from the measured driveline torque, so that no look-uptable needs to be established and no account of gear selection needs tobe made, can be very simple and (apart from the cost of the torquesensor itself) cheap.

Thus, in a system in accordance with the present invention, the trueoptimal relations can be used in an electronic braking system to assignthe optimal required braking forces on the brakes between the front andrear axles.

In addition to dynamic load measurements on front and rear axles, thetrue optimal relations will speed up the achievement of optimal tire toroad coefficient of adhesions during a stop, if used as reference.

In the case of a four wheel driven vehicle, the gear ratio in usebetween front and rear axle will determine the useable value of z_(i).1and z_(i).2.

The true optimum formulae could be used as a reference in the event thatonly one dynamic axle load is measured.

Referring now to FIG. 8, there is illustrated very diagrammatically oneway in which the principles of the present invention can be applied to aconventional hydraulic (i.e. non electronic) braking system of the typehaving a transverse engine and front wheel drive. Reference numeral 100indicates a conventional master cylinder operated by a foot pedal 102and providing a front/rear hydraulic split for simplicity ofexplanation. Thus, the master cylinder 100 supplies hydraulic fluidunder pressure via a line 106 to brake cylinders (not shown) at the rearwheels 104a, 104b. Positioned in the hydraulic line 106 is aconventional rear axle, load conscious apportioning valve which adjuststhe supply of hydraulic fluid to the rear brakes in dependence upon theload on the rear axle. Reference numeral 110 indicates the engine andgearbox of the vehicle which drives the front wheels 112a, 112b viarespective drive shafts 114a, 114b. The front brake cylinders (notshown) are connected to the master cylinder 100 via a hydraulic line 116which contains a valve 118 which is mounted between the vehiclechassis/frame (not shown) and an engine mounting member 120. Theconnection between valve 118 and the engine/gearbox 110 is arranged tobe such that when the engine is being accelerated or driven at constantspeed there is no effect on the valve at all and the valve simply passesthe hydraulic fluid from the master cylinder to the front brakes.However, when the engine is on overrun and is acting to decelerate thevehicle, the resulting rotation of the engine about its longitudinalaxis is arranged to operate the valve by a corresponding amount wherebythe degree of valve closure is dependent upon the level of the overruntorque on the drive shafts 114a, 114b. This arrangement thereforereduces the amount of front wheel braking for a given pedal effort bythe vehicle driver when the engine is operating under overrun(deceleration) conditions.

We claim:
 1. A system for achieving improvements to braking distributionin a multiple axle vehicle having a controlled braking system whichincludes foundation brakes operated in response to a foot pedal andwhich is subjected in use to at least one source of background brakingeffect which results in the vehicle experiencing a correspondingbackground braking force, comprising:means for measuring said backgroundbraking force resulting from said at least one source of backgroundbraking in the vehicle; and allocation means for dividing said measuredbackground braking force in a predetermined proportion between saidmultiple axles of the vehicle so that the respective axles are braked inconsideration of the divided background braking force.
 2. A systemaccording to claim 1, wherein, in order to assess total backgroundbraking force on the vehicle, the system includes means for measuringthe vehicle deceleration at a time when the vehicle is not being drivenand not yet being braked by its foundation brakes.
 3. A system accordingto claim 2, including means for enabling said deceleration measurementto be made at a point where foundation braking is signalled but beforethe resulting brake pressures have caused any significant brake forcesto be developed at said foundation brakes.
 4. A system according toclaim 3, wherein said means for measuring said background braking forcecomprises means for measuring vehicle deceleration just after firstmovement of said foot pedal whenever the foundation brakes are appliedand means for storing the measured deceleration level for use throughouta resulting stop.
 5. A system according to claim 3, including means forperforming said measurement of vehicle deceleration from the average ofreadings taken over a preset time period historical to the point atwhich the said foundation brakes are applied.
 6. A system according toclaim 1, including means for reducing said background braking level,assigned between said multiple axes of the vehicle, in a pre-programmedmanner as the speed of said vehicle falls.
 7. A system according toclaim 6, including means for normalizing, to a preset maximum speed forthe vehicle, said measured value for background vehicle deceleration,means for storing said measured value, means establishing a speedfall-off ratio table, and means for reducing the stored value as thevehicle speed falls on the basis of said speed fall-off ratio table. 8.A system according to claim 1, wherein, in order to measure totalbackground braking force on the vehicle, said system includes means formeasuring falling background deceleration of the vehicle, the resultingassessed background braking force being assigned by said allocationmeans between said axles of the vehicle in said predeterminedproportion, and wherein said system further comprises means forconverting said assigned braking forces into background brakingpressures and means for subtracting such braking pressures from eachcorresponding axle braking pressure calculated to meet a driver'sbraking demand.
 9. A system according to claim 1, including means forselecting vehicle drive line torque as said source of backgroundbraking.
 10. A system according to claim 9, including torque sensingmeans mounted around a drive shaft of the vehicle for measuring saiddrive line torque.
 11. A system according to claim 9, including meansfor measuring reaction forces at an engine mounting to provide saidmeasurement of drive line torque.
 12. A system according to claim 1, foruse in a hydraulic braking system of a vehicle having a transverselymounted engine and front wheel drive, including hydraulic valve meansdisposed in a hydraulic circuit leading to the front brakes and adaptedto respond to reaction of the engine on its mountings when the engine isin overrun operation so as to reduce the hydraulic pressure supplied tothe front brakes in dependence upon engine overrun torque.
 13. A systemfor achieving improvements to braking distribution in a multiple-axlevehicle having electronically controlled braking system which includesfoundation brakes operated in response to a foot pedal and which issubjected in use to plural sources of background braking effect whichresults in the vehicle experiencing a corresponding background brakingforce, comprising:means for measuring the total-background braking forceresulting from said sources of background braking in the vehicle; andallocation means for dividing said measured total background brakingforce in a predetermined proportion between said multiple axles of thevehicle, so that the respective axles are braked in consideration of thedivided background braking force.